1. Field of the Invention
The present invention relates to a direct-viewing type image display apparatus usable for various kinds of audio-visual (AV) apparatuses and office automation (OA) apparatuses.
2. Description of the Related Art
In order to realize the wide-spread popularization of a high-definition television set (HDTV) in the near future, a display apparatus is currently required to be more and more large-scale and to have a much larger capacity of image information. More specifically, in order to follow up the remarkable development of a high-resolution image, the number of the pixels in a display screen is required to be increased from 240 thousand, i.e., 400.times.600, up to a million, i.e., 1000.times.1000, or more and the size of the display screen is also required to be enlarged from a conventional type 20 to a type 40.
A conventional direct-viewing type TV using a Braun tube (or a cathode ray tube (CRT)) is insufficient for realizing such a large-scale display apparatus for home use. This is because a conventional direct-viewing type TV has problems in various aspects, i.e., the weight, the volume and the consuming power thereof. Therefore, it is desirable to develop a display apparatus allowing for solving such problems.
Various display apparatuses, other than a CRT like an emissive display apparatus such as a plasma display panel (PDP); a direct-viewing type liquid crystal display (LCD) apparatus using a back light; and a projection type display apparatus and the like are generally used. In order to fulfill the above-mentioned requirements, research has been actively conducted so as to develop various display apparatuses such as those described above.
Under such circumstances, a projection type display apparatus (or a projector), for realizing a large screen display by enlarging an image displayed in a CRT or an LCD panel so as to project the image on the screen, is now commercially available.
As an exemplary projection type display apparatus, a rear projection type display apparatus (or a rear projector) using a liquid crystal panel is disclosed in a document entitled "Display and Imaging" (Vol. 1, No. 1, p. 25, 1992). The display apparatus disclosed in the document will be described with reference to FIGS. 15 and 16.
FIG. 15 shows an optical system for a rear projection type display apparatus. This display apparatus includes: a light source (or a lamp) 101; a cold mirror 102; a projection optical system 110; and a projection lens 103 as main components thereof. The projection optical system 110 includes: dichroic mirrors 111 for separating and synthesizing spectrums; condenser lenses 112; and liquid crystal panels 113 for displaying three primary colors of red (R), green (G) and blue (B). In addition to these main components, the display apparatus further includes; two projection mirrors 104 and 105; and a transmission type screen 106.
In the display apparatus having the above-described configuration, an image is formed by the projection optical system 110 by using a light beam emitted from the light source 101 so as to be projected by the projection lens 103 via the projection mirrors 104 and 105 onto the screen 106. It is noted that a screen obtained by attaching a Fresnel lens substrate and a lenticular substrate is required to be used as the screen 106 in order to increase the resulting brightness of the image to be displayed.
FIG. 16 shows the disposition of these components inside the display apparatus. The main components, that is, the light source 101, the projection optical system 110, the projection lens 103 and the like are disposed in a lateral (or horizontal) direction on the upper bottom surface of the display apparatus. The light beam passed through the projection optical system 110 is emitted from the projection lens 103 in a lateral direction; refracted by a first projection mirror 104 at 90 degrees; and then directed to the rear side of the apparatus. The light beam being directed to the rear side is reflected by a second projection mirror 105, thereby projecting the image on the screen 106. By using this configuration, it is possible to make the apparatus thinner in size and lighter in weight.
On the other hand, an exemplary front projection type display apparatus (or a front projector) is disclosed in the documents entitled "Display and Imaging" (Vol. 1, No. 1, p. 47, 1992), and "Optical Engineering" (Vol. 31, No. 11, p. 2300, 1992) and the like.
The rear projection type display apparatus using a CRT or a liquid crystal panel has the following problems.
(1) It is necessary to employ a high-brightness projection CRT or a high-brightness projection liquid crystal panel (or a high-brightness projection lamp), so that the consuming power becomes disadvantageously high, e.g., 150 W or higher.
(2) Because of the adverse effects of the focusing characteristics of a Fresnel lens substrate and a lenticular substrate to be used for the screen, the resolution of the resulting image is reduced and the viewing angle at the time of display is restricted.
(3) A method in which an appropriate diffuser is provided on the screen may be used in order to enlarge the viewing angle for display. However, in such a case, the reduction of the display contrast and brightness (or the gains on the screen) can not be prevented.
In particular, in order to solve the problem (1) among those described above, it is desirable to develop a display apparatus consuming even lower power in view of the recent tendencies towards the protection of the global environment. For example, according to an energy saving promotion policy "Energy Star Program" issued by United States Environmental Protection Office, a power consumption of 30 W or lower is demanded. The problems (2) and (3) relate to the display quality of the image, and therefore these problem must be solved in order to represent an image of high quality with much presence on the screen of an HDTV.
In conventional display apparatuses, not only a rear projection type display apparatus but also an emissive display apparatuses such as a CRT and a PDP, and a front projection type display apparatus, if the display apparatus is placed within a bright environment or an environment where illuminated light is externally incident on the display screen of the apparatus, the ratio of the brightness of the displayed light to the surrounding light, i.e., a contrast, is reduced. Thus the brightness of the display apparatus is required to be increased in order to prevent the reduction of the contrast, so that the power consumption can not help being increased.